As a mechanism for performing a folding process on a paper along its fold line while conveying the paper, mechanisms according to Patent Documents 1 to 4, for example, are known. The mechanisms according to Patent Documents 1 and 2 are of so-called “buckle type”, and the mechanisms according to Patent Documents 3 and 4 are of so-called “knife type”.    Patent Document 1: Japanese Unexamined Utility Model Application Publication No. 58-180842    Patent Document 2: Japanese Unexamined Patent Application Publication No. 5-201610    Patent Document 3: Japanese Unexamined Utility Model Application Publication No. 63-65672    Patent Document 4: Japanese Unexamined Patent Application Publication No. 2005-41658